Conventionally, an image forming apparatus, such as a copying machine, a facsimile, a printer, a multifunction printer and the like, or a scanner and the like, transports a sheet, such as a paper, a document or the like by sandwiching the sheet between a pair of transportation rollers, which face each other, and rotating the transportation rollers.
Such transportation rollers are rotated by a driving force being transmitted from a driving source to a transportation roller gear via a drive gear so as to carry a sheet sandwiched between the transportation rollers in a direction in which opposed parts of the transportation rollers are moved.
In a configuration where a sheet is transported by rotation of transportation rollers, constant engagement of a drive gear and a transportation roller gear causes such a problem when a sheet is jammed that even if a user tries to remove the sheet, the removal of the sheet is difficult because a driving source prevents the transportation rollers from reversely rotating.
As a solution to such a problem, Patent Document 1 discloses two release mechanisms. In the release mechanism, an intermediate gear is provided between a drive gear and a driven gear. The release mechanism contacts the intermediate gear with the driven gear only at the time of drive, so as to transmit a driving force of the drive gear to the driven gear. At the time of nondrive, the release mechanism disengages the intermediate gear away from the driven gear.
In a first release mechanism, an intermediate gear is supported rotatably about a rotation center of a drive gear. When the drive gear is forwardly rotated, the intermediate gear engages with a driven gear by a rotation force generated by a pressure angle. When the drive gear is reversely rotated, the engagement of the intermediate gear with the driven gear is released by a rotation force generated by a pressure angle.
Furthermore, in a second release mechanism, an intermediate gear is supported rotatably about a rotation center of a drive gear. Further, the intermediate gear is provided with an elastic means for urging a pulling force in a direction where the engagement of the intermediate gear with the driven gear is released. When the drive gear is forwardly rotated, the intermediate gear engages with a driven gear by a rotation force generated by a pressure angle. When the drive gear is suspended, the engagement of the intermediate gear with the driven gear is released by a pulling force of the elastic means.
The arrangement in which the engagement of the intermediate gear with the driven gear is released at the time of nondrive enables a transportation roller provided on the driven gear to rotate reversely as well. This makes it possible to remove a sheet.
[Patent Document 1] Japanese Unexamined Patent Application Publication, Tokukaihei, No. 8-285034 (Published on Nov. 1, 1996)
However, the aforementioned release mechanism provided in the conventional drive transmission mechanism has the following problems.
In the first release mechanism, it is necessary to reversely rotate the driving source in order to release the engagement of the intermediate gear (drive gear) for transmitting the driving force from the driving source with the driven gear.
However, the driving source frequently transmits the driving force to the other units as well. Therefore, it is sometimes impossible to reversely rotate the driving source in connection with the other units. In such case, the first release mechanism cannot be adopted.
In the second release mechanism, on the other hand, it is unnecessary to reversely rotate the driving source. However, there is a problem that the driving force from the driving source is impaired because the mechanism is configured to constantly resist the pulling force of the elastic means so as to transmit the driving force to the driven gear.